As shown by K. D. Strange and J. A. Hoffer, “Gait Phase Information Provided by Sensory Nerve Activity During Walking: Applicability as State Controller Feedback for FES”, IEEE Transactions on Biomedical Engineering, vol. 46, no. 7, pp. 797–809, July 1999, the natural sensory signals recorded by nerve cuff electrodes can be used as a reliable source of feedback for closed-loop control of Functional Electrical Stimulation (FES) devices. As other examples, bladder pressure can be measured to control micturition, or sensory feedback from touch sensors in the skin can be recorded for improved activation of paralyzed limb muscles; see, for example, T. Sinkjaer et. al., “Electroneurographic (ENG) Signals from Intradural S3 Dorsal Sacral Nerve Roots in a Patient with SupraSacral Spinal Cord Injury”, Proceedings of the 5th Annual Conference of the International Functional Electrical Stimulation Society, pp. 361–364, Denmark, June 2000; and M. K. Haugland and J. A. Hoffer, “Slip Information Provided by Nerve Cuff Signals: Application in Closed-Loop Control of Functional Electrical Stimulation”, IEEE Transactions on Rehabilitation Engineering, vol. 2, no. 1, pp. 29–36, March 1994, respectively.
Recording nerve cuff electrodes were introduced as a research tool in the 1970's and the first human trials started in the 1990's. See J. A. Hoffer and K. Kallesøe, “How to Use Nerve Cuffs to Stimulate, Record or Modulate Neural Activity”, Neural Prostheses for Restoration of Sensory and Motor Function, Chapter 5, CRC Press, 2000.
Several different nerve signals are needed to control a prosthetic device; see U.S. Pat. No. 4,750,499 to Hoffer for a “Closed-Loop, Implanted-Sensor, Functional Electrical Stimulation System for Partial Restoration of Motor Functions”. The signals picked up by the nerve cuff electrodes have been recorded mainly using commercially available components located externally to the body, which consume a lot of area and a lot of power. Such recording setups require long leads that course transcutaneously. This increases the risk of infection for the patient. Long cables also increase the risk of wire breakage and reduce portability. The added resistance and capacitance of long cables can contribute to signal shunting and greater pick-up of unwanted signals (e.g. stimulation artifacts and power-line influx noise).
The biggest problems in recording neural signals using nerve cuff electrodes are the very low signal amplitude and low signal-to-noise ratio (S/N) that are characteristic of these signal sources. When dealing with signals in the μV range, minimization of the preamplifier noise is of extreme importance. Under such conditions, it is useful to passively boost the signal amplitude with an audio transformer before it encounters the first active amplification stage. This has been a common practice in recording neural signals. See, for example, Z. M. Nikolić et. al., “Instrumentation for ENG and EMG Recordings in FES Systems”, IEEE Transactions on Biomedical Engineering, vol. 41, no. 7, pp. 703–706, July 1994. The smallest audio transformers in the market have an area of around 1 cm2. Since several transformers would be needed in a multi-channel recording device, this solution is impractical.
Recently, N. Donaldson et al. described an implantable, single-channel cuff-recording system, fabricated using discrete components, which uses no input transformer. See N. Donaldson et al., “An Implantable Telemeter for Long-Term Electroneurographic Recordings in Animals and Humans”, Proceedings of the 5th Annual Conference of the International Functional Electrical Stimulation Society, pp. 378–381, Denmark, June 2000. This design, although useful for research purposes, has limitations in its clinical applicability due to its size and power requirements, and is not suitable for clinical applications where several recording-channels are needed.
Various designs of custom-integrated amplifiers for neural activity recording have been presented throughout the years. However, these designs have generally dealt with neural source voltage amplitudes at least two orders of magnitude larger than the signals that are typically recorded using nerve cuff electrodes.
K. Papathanasiou and T. Lehmann, “An Implantable CMOS Signal Conditioning System for Recording Nerve Signals with Cuff Electrodes”, Proceedings of the IEEE International Symposium on Circuits and Systems, pp. V 281–284, Switzerland, May 2000 discloses an integrated amplifier. There is no mention of noise levels and the amplifier gain is of the open-loop type. Open-loop amplifiers present several problems associated with their high gain. Two of the main problems are potential saturation due to intrinsic amplifier offset and muscular activity (EMG) of much higher amplitude than the nerve signals of interest, requiring the amplifier to utilize special circuit techniques to compensate for these effects. Furthermore, FES systems based on closed-loop feedback control typically require amplifiers having variable gain.
There is a need for an amplifying circuit that is suitable for use in implantable devices. There is a particular need for such a circuit which can provide several externally controllable gain levels, as required for the development of FES systems based on closed-loop feedback control.